The ecology of the free-living stages of Ostertagia ostertagi in a winter rainfall region

Abstract

On a series of pasture plots, 500 g pats of cattle faeces containing eggs of the trichostrongylid Ostevtagia ostertagi were deposited at intervals of about 6 weeks from June 1976 to June 1977. The development and survival of eggs and larvae in dung pats and the survival of larvae on surrounding herbage and soil were measured and expressed as a percentage of the initial population of eggs. During summer months comparisons were made between the pats on irrigated and those on non-irrigated plots. Minimum temperatures within dung pats were always greater than those required for development, and infective larvae were recovered from all plots. The onset of development was delayed from 3 to 12 days in summer and from 34 to 68 days in winter, which led to a wide distribution of developmental stages. Rates of development were closely related to the temperatures in soil and dung pats, and mortality rates of pre-infective stages increased very rapidly with increasing temperatures and decreasing moisture levels. Infective larvae were present in abundance on herbage and in soil between 6 and 10 weeks after deposition, provided that the moisture content of these was high. Mortality of these larvae was low during winter and early spring but increased rapidly in mid- to late spring, irrespective of the time of deposition of dung pats. Mortality rates of infective larvae in dung pats deposited in either spring or summer were low and a large proportion of these larvae were capable of moving into herbage and soil after autumn rains. Irrigation during summer did not provide ideal conditions for the development of infective larvae from eggs, but hastened larval migration from dung pats.